Grid files are important for shifting and transforming between datums
US, Canadian, French and New Zealand¶
- http://download.osgeo.org/proj/proj-datumgrid-1.6.zip: US, Canadian, French and New Zealand datum shift grids - unzip in the nad directory before configuring to add NAD27/NAD83 and NZGD49 datum conversion
Background in ticket #145
We basically have two shift grids available. An official here:
And a derived in a temporary location which is probably going to disappear soon.
Main problem seems to be there’s no mention of distributivity of the grid from the official website. It just tells: “you can use freely”. The “contact” link is also broken, but maybe someone could make a phone call to ask for rephrasing that.
With the support of i-cubed, Frank Warmerdam has
written tools to translate the HPGN grids from NOAA/NGS from
into NTv2 format for convenient use with PROJ.4. This project included
implementing a .los/.las reader
for GDAL, and an NTv2 reader/writer.
Also, a script to do the bulk translation was implemented in
The command to do the translation was:
loslas2ntv2.py -auto *hpgn.los
As GDAL uses NAD83/WGS84 as a pivot datum, the sense of the HPGN datum shift offsets were negated to map from HPGN to NAD83 instead of the other way. The files can be used with PROJ.4 like this:
cs2cs +proj=latlong +datum=NAD83 +to +proj=latlong +nadgrids=./azhpgn.gsb +ellps=GRS80
# input: -112 34
# output: 111d59'59.996"W 34d0'0.006"N -0.000
This was confirmed against the NGS HPGN calculator.
The grids are available at http://download.osgeo.org/proj/hpgn_ntv2.zip
HTPD describes similar grid shifting
Not all grid shift files have licensing that allows them to be freely distributed, but can be obtained by users through free and legal methods.
Although NTv1 grid shifts are provided freely with PROJ.4, the higher-quality NTv2.0 file needs to be downloaded from Natural Resources Canada. More info: http://www.geod.nrcan.gc.ca/tools-outils/ntv2_e.php.
Visit the NTv2, and register/login
Follow the Download NTv2 link near the bottom of the page.
Unzip ntv2_100325.zip (or similar), and move the grid shift file NTV2_0.GSB to the proj directory (be sure to change the name to lowercase for consistency) * e.g.: mv NTV2_0.GSB /usr/local/share/proj/ntv2_0.gsb
- Test it using:
cs2cs +proj=latlong +ellps=clrk66 +nadgrids=@ntv2_0.gsb +to +proj=latlong +ellps=GRS80 +datum=NAD83 -111 50
111d0'3.006"W 50d0'0.103"N 0.000 # correct answer